Field of the Invention
The invention relates to a method for anticipatory or pilot control of a switched-mode power supply for compensating for fluctuations in a feed voltage, wherein the switched-mode power supply includes at least one electronic switch and a control circuit with a frequency generator, a comparator circuit, and a trigger circuit or multivibrator; the trigger circuit or multivibrator furnishing a trigger signal for the at least one electronic switch as a function of an output signal of the comparator circuit and an output signal of the frequency generator; the comparator circuit having a switching threshold being defined as a function of a load current of the switched-mode power supply; the comparator circuit having a signal input being triggered with a periodically rising and falling control signal having a frequency defined by the frequency generator; and the control signal having a signal shape being varied as a function of an instantaneous value of a feed voltage of the switched-mode power supply.
The invention also relates to a control circuit configuration for a switched-mode power supply having at least one electronic switch, for compensating for fluctuations in a feed voltage, including a frequency generator, a comparator circuit, a trigger circuit or multivibrator, and a circuit element furnishing a periodically rising and falling control signal with a frequency defined by the frequency generator; the trigger circuit or multivibrator furnishing a trigger signal for the at least one electronic switch as a function of an output signal of the comparator circuit and an output signal of the frequency generator; the comparator circuit having a switch threshold being defined as a function of a load current of the switched-mode power supply; the comparator circuit having a signal input being triggered with the periodic control signal; and the control signal having a signal shape being varied as a function of an instantaneous value of the feed voltage of the switched-mode power supply.
Both single-phase and push-pull switched-mode power supplies are known, especially with integrated control circuits in which anticipatory control is provided to compensate for fluctuations in the feed voltage. Commercially available integrated control circuits TDA4700, TDA4718, TDA4716, TDA4714, TDA4918 and TDA4919, and their use in switched-mode power supplies, are described, among other sources, in the Siemens 1989/90 data manual "ICS fur Industrielle Anwendungen" [ICs for Industrial Applications], pp. 182-231 and 242-260. Such switched-mode power supplies are also shown in the reprint of Siemens Components 5/88, pp. 191-194; 6/88, pp. 260-265; and 1/89, pp. 12-17, by R. Blockl, which appeared under the title "TDA 4918/4919 -- New Generation of Control ICs for Switched-Mode Power Supplies" especially in FIG. 3 of Part 2. In all of those control circuits, a pulse width modulated signal is furnished in order to trigger the electronic switches of switched-mode power supplies constructed from them. The pulse width is defined both by an output voltage regulator circuit and by an anticipatory control that takes the feed voltage into account. The integrated circuits described therein include, among other elements, a frequency generator, a ramp generator, a comparator that compares the output signal of the ramp generator with a voltage level defined as a function of the instantaneous value of the output voltage of the switched-mode power supply (and therefore of the load current), and a flip-flop that is triggered by the frequency generator and the aforementioned comparator and thus for normal operation defines the pulse width of the trigger pulses for the electronic switch or switches of a switched-mode power supply constructed with the trigger circuit. The slope of the leading edge of the output signal of the ramp generator can be varied by varying a control current.
In the known switched-mode power supplies, a method for anticipatory control is used that serves to compensate for feed voltage fluctuations. A variation in the feed voltage causes a variation in the slope of the leading edge of the ramp generator. If the feed voltage is high, the slope of the leading edge of the ramp generator output signal is greater, so that the threshold value of the comparator, which is affected by the output voltage controller, wherein the comparator is connected downstream of the ramp generator, is reached sooner than if the feed voltage is lower and consequently if the ramp course is flatter. Within certain limits, a variation in the feed voltage accordingly does not need to be stabilized by the regulator. The regulator is provided to keep the output voltage constant and the stabilization would be performed by adapting the threshold value of the comparator.
The known anticipatory control method, in which the ramp slope is controlled, is useful for feed voltages that do not deviate greatly from a nominal value, with an example being rectified sinusoidal voltages that are partially smoothed by a capacitor. Since the slope of the leading edge of a ramp generator output signal cannot be varied arbitrarily, and in particular cannot become arbitrarily great, as would be necessary in the event of major deviations of the feed voltages of a mean value, known switched-mode power supplies accordingly require either preparation of the feed voltage, for instance with the aid of a capacitor, or stabilization of fluctuations in the feed voltage by means of the regulator that keeps the output voltage constant.
Particularly for anticipatory control of an input voltage in the form of an unsmoothed rectified sinusoidal voltage, a correct specification in the region of the zero crossover and in the region of the amplitude is also important. Good anticipatory control over the entire operating range by means of a controlled ramp slope cannot be attained in that case. As will be explained below in conjunction with FIG. 4, there are limits to a method for anticipatory control of a switched-mode power supply with the aid of a variable ramp slope. An anticipatory control by means of controlled ramp slope functions properly only with input voltages that are in the vicinity of the rated value for which the anticipatory control was dimensioned.